Two highly important uses of heat energy obtained with nuclear power are the generation of electrical power and the generation of industrial process heat.
As for the generation of electrical power, to date the art's focus has been on large monolithic water-cooled reactors with the exception of the large carbon dioxide cooled designs used in the United Kingdom. Such water-cooled reactors have a maximum temperature of operation of about 600.degree. F., which severely limits their utility for industrial process heat applications (which are normally carried out at higher temperatures). Furthermore, even in their use for electrical power generation, large monolithic water-cooled reactors exhibit substantially disadvantageous characteristics. Examples are the great cost of maintaining acceptably safe operating conditions, and the threat of possible exposure to radiation of humans in the vicinity of the reactor. These characteristics significantly detract from the usefulness of such reactors in generating electrical power. The great cost of maintaining safe operating conditions substantially undercuts the economic viability of using such reactors. Also, the comparably substantial cost of protecting humans from radiation exposure also has an adverse impact on operating and maintenance costs.
Further, as can be appreciated by the skilled worker, when a water-cooled nuclear reactor is subjected to loss of forced cooling the heat-up rate of the reactors' metallic-clad fuel due to heat-energy input (decay heat from reactor's fuel, stored heat, and heat from the metal-water reaction) is exceedingly rapid. Temperatures resulting in cladding failure are reached in a matter of seconds with the consequent release of fission products from the reactor's fuel elements.
As an alternative to the problematical large monolithic water-cooled reactors, the art can turn to large high-temperature helium gas-cooled graphite-moderated nuclear reactors. These reactors are an advance beyond carbon dioxide gas cooled graphite-moderated designs whose temperature is constrained by the properties of carbon dioxide. They have the advantage that they can be operated at temperatures up to about 1750.degree. F., which permits their use under conditions normally required for efficient electric power generation, and otherwise meets a wide range of industrial process heat generation requirements.
These large high-temperature helium gas-cooled graphite-moderated reactors, in contrast to the water-cooled variety discussed above, have a much slower heat-up rate. Temperatures at which the danger of fission product release is posed are reached only after the passage of several hours from the time of loss of forced cooling. Furthermore, since metal cladding is not employed in these high-temperature reactors to effect fission product retention, the danger of metal cladding failure at high temperature is obviated. Rather than rapid and catastrophic failure, the failure mode encountered in such high-temperature reactors is gradual, initially constituting a slow diffusion, through a ceramic barrier, of fission products from the fuel particles which are found in the reactor's core (geometric stability of the elements is maintained).
Nonetheless, the high-temperature helium gas-cooled reactors discussed in the preceding paragraphs are also subject to some shortcomings. First, even though these reactors are more resistant than water-cooled reactors to the effects of loss of forced cooling, they too reach a state at which fission products can be released from the reactor core due to uncontrolled heat-up. Additionally, due to their large size, these reactors cannot be fabricated in a controlled factory environment, but must be assembled on-site in the field. This requirement introduces a significant increase in the cost of reactor fabrication. Moreover, each large reactor must be tailor-made for the application to which it is put and the locale in which it is situated. This factor also increases the cost of reactor fabrications substantially, as standardization of design is rendered infeasible.
Thus, the state of the art leaves much to be desired.